198 A HISTORY OF BIRDS 



tracery of white over a deep blue ground. Nothing appears 

 to be known as to the possible use of this chalky layer, but in 

 one case at least a possible explanation may be found. This 

 case is furnished by the Razor-bill, but the interpretation offered 

 may be more conveniently considered later (p. 209). 



The thickness of the shell varies within certain limits ; thus 

 in some cases, especially among the smaller birds, the shell is 

 so thin that the egg becomes translucent, as in the case of the 

 Kingfisher, while among the Ostrich tribe it is relatively thick. 



While as a rule the surface of the egg is smooth, in some 

 cases, especially among the Ostriches, the egg is deeply pitted, 

 the Apteryx and Tinamous excepted. 



It has been stated that the texture and colour of the shell of 

 eggs of hybrids can be distinguished from those of the pure 

 species. It is conceivable that this may be so, but the conten- 

 tion that the eggs of a hen mated with a cock of another 

 species are similarly affected is lacking in support, and will 

 almost certainly prove to be wrong, for it must be remembered 

 that the shell is a purely extraneous product, that is to say, it is 

 a secretion of the glands of the oviduct, and has nothing to do 

 with the germ cells, which are alone affected by such crossing. 



We must pass now to the colours of eggs, or rather, of the 

 shells thereof First of all, let us dismiss certain technical but 

 important facts concerning the composition of these colours. 

 We are indebted for most of what we know of the pigments 

 colouring eggs to the late Dr. H. C. Sorby who by spectrum 

 analysis differentiates seven distinct colours : — 



1. Oorhodeine — a very permanent, and very common, red- 



brown pigment. 



2. Oocyan j blue pigments probably closely related, 



3. Banded oocyan [ though the second only yields a 



banded spectrum. 



4. Yellow ooxanthine— a bright yellow pigment which, 



mixed with oocyan gives the 

 beautiful green colour seen in 

 the egg of the Emu. 



5. Rufous ooxanthine — a reddish-yellow pigment possibly 



peculiar to the eggs of the Tin- 

 amous. 



